This invention relates to arborescent or highly branched block copolymers comprising branched soft segments with a low glass-transition temperature (Tg) and hard segments with a high Tg or crystalline melting point that exhibit thermoplastic elastomeric properties. More particularly this invention relates to highly branched block copolymers of polyisoolefins and polymonovinylidene arenes that exhibit thermoplastic elastomeric properties.
Thermoplastic elastomers (TPEs) are polymeric materials, which combine the properties of vulcanized rubbers and the processability and recylability of thermoplastics, see for example B. M. Walker, xe2x80x9cHandbook of Thermoplastic Elastomersxe2x80x9d, Van Nostrand Reinhold, New York (1979). While blends of elastomers and plastics are not compatible and show gross phase separation, block copolymers can only phase separate on a microscopic scale due to the connectivity of elastomeric and plastic blocks.
Branched polymers are of commercial interest due to their having markedly lower viscosity and less shear sensitivity than their linear counterparts. Thus branched block copolymers that have the added benefit of being thermoplastic elastomers should have a wide variety of commercial applications depending upon the elastomer and the thermoplastic used to form the block copolymer.
About 40% of TPEs are block copolymers, which contain both soft segments with a low glass-transition temperature (Tg) and hard segments with a high Tg or crystalline melting point, see G. Holden, in xe2x80x9cRubber Technologyxe2x80x9d, ed. M. Morton, Van Nostrand Reinhold, New York, Ch. 16, 465 (1987). The hard segments associate, leading to physical crosslinks, which disappear when heated above a certain temperature (Order-Disorder Temperature, ODT) and reappear immediately on cooling. The hard phase determines the mechanical strength, heat resistance, upper service temperature and strongly affects the oil and solvent resistance of a TPE. The chemical nature of the soft segments has an influence on elastic behavior, low temperature flexibility, thermal stability and aging resistance. According to present understanding in the field, in order to get good phase separation in block-type TPEs leading to good mechanical properties, the length of the elastomer chains should be as uniform as possible. This can be achieved by living polymerization, a unique process without termination and other side reactions of the growing polymer chain. Living conditions producing relatively uniform polymers can be achieved in anionic, cationic and radical systems.
An important commercial example of thermoplastic elastomeric block copolymers is styrene-elastomer-styrene, produced by living anionic polymerization. Most of the styrenic block copolymers have the general formula S-E-S, where S represents a hard amorphous polystyrene block and E represents a soft elastomeric block. Many of the polystyrene-polydiene block copolymers that are TPEs have the basic structure poly(styrene-block-butadiene-block-styrene) (S-B-S) or poly(styrene-block-isoprene-block-styrene) (S-I-S). The applications of these block copolymers are numerous. Important applications include solvent based and hot melt adhesives, sealants, coatings, hose, asphalt modifiers and sporting goods and automobiles, see G. Holden, N. R. Legge, R. Quirk, H. E. Schroeder (Eds.), xe2x80x9cThermoplastic Elastomersxe2x80x94A comprehensive Reviewxe2x80x9d, Hanser Publishers, Munich (1996) and G. Holden, in xe2x80x9cEncyclopedia of Polymer Science and Engineeringxe2x80x9d, ed. J. I. Kroschwitz, John Wiley and Sons, New York, Vol. 5, 416 (1996).
Recently, TPEs from another class of styrenic block copolymers have been developed with polyisobutylene (PIB) elastomeric segments, see U.S. Pat. No. 4,946,899 issued to J. P. Kennedy et al. FIGS. 1A and 1B show a diagrammatic representation of the first generation of these PIB-based TPEs which are linear triblock (FIG. 1A) and triarmxe2x80x94star block structures shown in FIG. 1b. An important advantage of these TPEs based on polyisobutylene-polystyrene (S-IB-S) block copolymers is that there is no need of hydrogenation of the elastomeric segments like that in the case of S-B-S or S-I-S, because of the presence of a saturated PIB elastomeric block. These novel TPEs were found to have excellent damping characteristics (similar to butyl rubber over a wide frequency range), oxidative and hydrolytic stability and good gas barrier properties, see K. Koshimura, H. Sato, Polym. Bull, 29, 705 (1992) and J. P. Puskas, G. Kaszas, Rubber Chem. Technol., 66, 462 (1996).
The first generation of these TPEs were linear and triarm-star blocks, whose synthesis and basic chemical characterization have been reported, see G. Kaszas, J. E. Puskas, W. G. Hager and J. P. Kennedy, J. Polym. Sci., Polym. Chem., A29, 427 (1991).and J. E. Puskas, G. Kaszas, J. P. Kennedy, W. G. Hager, J. Polym. Sci., Polym. Chem., A30, 41 (1992). The living polymerization of IB by di- and tri-functional initiators gives a uniform rubbery mid-block, followed by the sequential addition of styrene (St) which results in a glassy outer block.
The architecture of copolymers can be controlled by the synthesis procedure, and TPEs with various composition and molecular weight (MW) have been prepared and characterized; for a review see J. P. Puskas, G. Kaszas, Rubber Chem. Technol., 66, 462 (1996). The most frequently used initiators are di- and tricumyl derivatives, especially di- and tricumyl-ether and -chloride. The co-initiator mainly used for making high molecular weight PIBs, suitable for block copolymer synthesis, is TiCl4. The control of living IB polymerization is further improved by the use of electron pair donors like dimethyl sulfoxide or dimethyl acetamide and a proton trap such as di-tert-butyl pyridine (DtBP). These additives lead to better control of IB polymerization, resulting in narrow molecular weight distribution (MWD) PIB, and also improve the blocking efficiency of St monomers during TPE synthesis.
The S-IB-S triblocks and three-arm radial blocks obtained in the absence of electron pair donor and/or proton trap exhibit poor tensile properties due to inefficient blocking. The process has successfully been scaled up to a lb/batch scale (G. Kaszas, Polym. Mater. Sci. Eng., 67, 325 (1992) and an inventory of block copolymers have been prepared and characterized (P. Antony, J. E. Puskas: Proceedings of the Polymer Processing Society Meeting, May 21-24, Montreal, Canada (2001). Kuraray Inc., Japan, recently test-marketed linear S-IB-S block copolymers. The important emerging applications of these first generation linear and tri-star block copolymer materials include medical applications such as rubber stoppers for drugs and blood, gaskets and caps for syringes, blood and drug storage bags and tubes and the like as disclosed in Japanese Patent No. 5,212,104; Japanese Patent No. 5,269,201; and Japanese Patent No. 5,295,054.
Due to the high oxidative and chemical stability of the S-IB-S macromolecules, these materials have the potential to replace silicone rubber or other soft to semirigid bio-implantable polymers; U.S. Pat. Nos. 5,741,331 (1998); 6,102,939 (2000); and 6,197,240 (2001) issued to Pinchuk, L. Pinchuk, I. J. Khan, J. B. Martin and G. J. Wilson: Polyisobutylene-Based Thermoplastic Elastomers For Ultra Long Term Implant Applications; Sixth World Biomaterials Congress Transactions, 1452 (2001), and Pinchuk, L., Khan, I. J., Martin, J. B., Bridgeman, J., Wilson, G. J., Glass, J., Si, J. and Kennedy, J. P.; A New Family of Thermoplastic Elastomers for Ultra-Long Term Implant Based Upon a Backbone of Alternating Quaternary and Secondary Carbonsxe2x80x9d; 24th Annual Meeting of the Society for Biomaterials; April 22-26, San Diego, (1998), p. 173; are directed to biologically stable, non-biodegradable implant devices and methods of producing same, using linear or star polyolefin copolymers having the structures shown in FIG. 1. They also disclosed the structure shown in FIG. 2A.
The second generation of PIB-based TPEs shown in FIG. 2A are an even more recent development and include multi-arm-star blocks (S. Jacob, J. P. Kennedy, Advances in Polym. Sci, 146, 1 (1999); J. E. Puskas, W. Pattern, P. M. Wetmore, V. Krukonis, Rubber Chem. Technol. 72, 559 (1998). Multiarm-stars are expected to have improved strength and shear stability. Three-arm stars were shown to have considerably higher strength than comparable linear triblocks, but their shear sensitivity is still a concern; shearing off an arm would lead to a linear triblock. For polymers prepared by anionic polymerization beyond six arms no change in properties was found (L. K. Bi, L. J. Fetters, G. Quack, R. N. Young xe2x80x9cThe Synthesis, Rheology and Morphology of Star Block Copolymersxe2x80x9d, Rubbercon 77, Int. Rubber Conf., Akron, Ohio, Paper No. 11, 11 (1977).
Although there are several reports on the synthesis and characterization of linear triblock, three-, six-, octa- and multi-arm (10-60) star S-IB-S block copolymers in the literature, the effect of hard and soft segment composition and molecular architecture on the mechanical properties of these TPEs has not been studied systematically. It has been reported that the minimum PS molecular weight required for good phase separation and physical properties is around 8000, and the maximum tensile strength achieved was 17-24 MPa with 400-600 % elongation, depending on the measurement method (for reviews see J. P. Puskas, G. Kaszas, Rubber Chem. Technol., 66, 462 (1996) and J. E. Puskas, G. Kaszas, Progr. Polym. Sci., Elsevier Science Ltd., 25(3), 403 (2000)).
Referring again to U.S. Pat. No. 4,946,899, this patent discloses a thermoplastic elastomeric star-shaped block copolymer comprising a polyisobutylene midblock and endblocks of polymerized styrene wherein the ratio of the the weight average molecular weight to the number average molecular weight of the midblock is from about 1.01 to about 1.5. It is further disclosed that each branch of the aforesaid star-shaped block copolymer is of essentially equal length.
U.S. Pat. No. 5,428,111 issued to Faust et al. discloses a method of producing block copolymers having polyolefin midblocks and styrenic end blocks by the living polymerization of aromatic styrenic monomers initiated from living polyisoolefin chain end.
U.S. Pat. No. 5,458,796 issued to Storey et al. discloses a process for the synthesis of star polymers, specifically polyisobutylene star-branched polymers by living carbocationic polymerization of isobutylene homopolymers, random copolymers or block copolymers using a monofunctional initiator and subsequently linking the preformed arms using divinylbenzene or diisopropenylbenzene.
U.S. Pat. No. 5,721,331 issued to Shachi et al. is directed to a process of producing isobutylene-based block copolymers composed of isobutylene units and styrene blocks. This patent discloses that, if one uses the process disclosed in U.S. Pat. No. 4,946,899 issued to J. P. Kennedy et al., and continues the polymerization after the styrene monomer is consumed, the active living chain ends can attack the styrene block of another chain, creating multiblocks shown in FIG. 1C. This reaction, coupling via electrophilic substitution on the aromatic rings of the PS blocks, is well-known (Fodor Zs, Gyor M, Wang HC, Faust R. J Macromol Sci, Pure Appl Chem A 1993;30(5):349-63). Shachi claimed that this chain coupling, evidenced by multimodal molecular weight distribution (termed as xe2x80x9ccontinuous distributionxe2x80x9d in U.S. Pat. No. 5,721,331 where the GPC peaks correspond to multiplets of the Mn of the starting linear triblock material), leads to improved properties, namely higher tensile strength and lower tensile set values. Similar multiblocks, consisting of polyisobutylene rubbery segments and cyclized polyisoprene hard segments and exhibiting thermoplastic elastomeric properties, are disclosed in U.S. Pat. No. 4,910,261 issued to G. Kaszas, J. E. Puskas and J. P. Kennedy. These materials are a mixture of linear chains and various multiblocks. In the aforementioned two patents the polyisobutylene rubbery blocks are linear. The branched structure of the materials is the result of branching of the outer plastic blocks of the TPE.
Kee and Gauthier in Macromolecules, 32, 6478 et seq. (1999) describe the preparation of highly branched polystyrene-polyisoprene copolymers by the successive grafting of the polymeric building blocks (graft on graft) which have a well-defined structure and the average molecular weight distributions of both the highly branched polystyrene and the polystyrene-polyisoprene graft copolymer are very narrow, being 1.1 or less. U.S. Pat. No. 6,156,859 issued to Langstein et al. discloses a process for producing highly branched polyisoolefins by the reaction of isoolefins by polymerizing an isoolefin in the presence of a multfunctional monomer and an alkylalumoxane at a temperature between 20xc2x0 C. and xe2x88x92100xc2x0 C.
Puskas et al. reported the synthesis and characterization of arborescent polyisobutylenes by copolymerising an inimer with isobutylene (Makromol. Chem, Macromol. Symp. 132, 117 (1998); Proceedings of the World Polymer Congress (IUPAC Macro 2000), 384 (2000)). Arborescent (hyperbranched) polymers belong to the class of dendritic polymers but are characterized by an irregular tree-like structure. These polymers are a relatively recent development and very little structure-property relationship data is available, see D. A. Tomalia: Makromol. Chem., Macromol. Symp. 101, 243 (1996).
Heretofore there has not been disclosed an arborescent branched block copolymer with thermoplastic elastomeric properties comprising a highly branched polyisoolefin block wherein the branches are of irregular length in which some of the branches of the aforesaid polyisoolefin bear rigid polymer end block segments. Thus, it would be very advantageous to provide such materials that have thermoplastic elastomeric properties comparable, or superior, to the materials shown in FIG. 2A but which can be produced more economically than these materials. Such materials would have many uses including use as implantable prostheses in medical applications.
The present invention provides an arborescent branched block copolymer, comprising an arborescent elastomeric polymer block having more than one branching point, the arborescent elastomeric polymer block having a low glass-transition temperature (Tg), and some or all of the branches of the arborescent elastomeric polymer block being terminated in rigid polymer block segments with a high Tg or crystalline melting point, the arborescent branched block copolymer exhibiting thermoplastic elastomeric properties.
The present invention also provides an arborescent branched block copolymer of a polyisoolefin and a polymonovinylidene arene comprising an arborescent elastomeric polyisoolefin polymer block having more than one branching point, and some or all of the branches of the aforesaid arborescent polyisoolefin polymer block terminate in polymonovinylidene arene plastic endblocks, the arborescent branched block copolymer having thermoplastic elastomeric properties.
The present invention also provides an arborescent branched block copolymer of polyisobutylene and polystyrene comprising an arborescent elastomeric polyisobutylene polymer block having more than one branching point and some or all of the branches of the aforesaid arborescent polyisobutylene polymer block being terminated with polystyrene endblocks, the arborescent branched block copolymer having thermoplastic elastomeric properties.
The present invention also provides a process for producing an arborescent branched block copolymer of a polyisoolefin and a polymonovinylidene arene, comprising:
a) polymerizing an isoolefin in the presence of at least one inimer and a Lewis acid halide coinitiator, at a temperature of between about xe2x88x9220xc2x0 C. and xe2x88x92100xc2x0 C. to produce an arborescent elastomeric branched polyisoolefin polymer having more than one branching point, the inimer including at least one group for (co)polymerizing in a cationic polymerization of the isoolefin and at least one group for initiating cationic polymerization of the isoolefin; and thereafter
b) adding a compound having an effective electron pair donor for improving blocking efficiency and adding a monovinylidene arene suitable for the production of the polyvinylidene plastic blocks with some or all of the branches of the aforesaid arborescent elastomeric branched polyisoolefin polymer being terminated with polymonovinylidene arene plastic endblocks; and
c) terminating said polymerization reaction after a selected period of time by addition of an effective nucleophile compound which terminates said polymerization reaction, said selected period of time being sufficiently long enough to ensure production of at least individual units of said arborescent branched block copolymer, the arborescent branched block copolymer of a polyisoolefin and a polymonovinylidene arene having thermoplastic elastomeric properties.
In the process the inimer, a compound carrying both an initiator and a monomer functionality (IM), is copolymerized with one or more olefins. Very high MW arborescent PIBs are produced using 4-(2-hydroxy-isopropyl) styrene and 4-(2-methoxy-isopropyl) styrene as IM in a xe2x80x9cone-potxe2x80x9d living-type polymerization system. The reactive chain ends of arborescent PIB are blocked with a material such as stryrene to form PIB-PS blocks.
In another aspect of the invention there is provided a process for producing an arborescent branched polyisoolefin block copolymer, comprising:
a) polymerizing an isoolefin in the presence of an inimer and a Lewis acid halide coinitiator at a temperature of between about xe2x88x9220xc2x0 C. and xe2x88x92100xc2x0 C. to produce an elastomeric arborescent branched polyisoolefin polymer having more than one branching point, the the inimer having a formula 
where R can be H, OH3 or an alkyl or aryl group; thereafter
b) adding a compound having an effective electron pair donor for improving blocking efficiency and adding a monovinylidene arene suitable for the production of the polyvinylidene arene plastic endblocks; and
c) terminating said polymerization reaction after a selected period of time by addition of an effective nucleophile compound which terminates said polymerization reaction, said selected period of time being sufficiently long enough to ensure production of at least individual units of said elastomeric arborescent branched block copolymer with some or all of the branches of the aforesaid elastomeric arborescent polyisoolefin polymer block terminated in polymonovinylidene arene plastic endblocks, the arborescent branched block copolymer having thermoplastic elastomeric properties.
The present invention also provides an arborescent branched block copolymer comprising an arborescent elastomeric polymer block having more than one branching point, the arborescent elastomeric polymer block having a low glass-transition temperature (Tg), and some or all of the branches of the arborescent elastomeric polymer block being terminated in rigid polymer block segments with a high Tg or crystalline melting point the arborescent branched block copolymer exhibiting thermoplastic elastomeric properties formed into an article of manufacture comprising an implantable prosthesis or implant.